At the present time, a thin steel sheet having a sheet thickness of 1.4 to 5 mm is produced as a hot-rolled steel sheet by using, as a starting material, a slab having a thickness exceeding 200 mm and subjecting the material to hot rolling. In the above current process, the basis of the technique for formation of an intended structure in the present saturation, that is, the regulation of the structure, is to increase the number of nucleation sites in transformation by causing recrystallization in the material in the step of hot-rolling the material to refine a coarse austenitic structure to increase the intergranular area or by rolling the material in a non-recrystallization region to introduce a deformation zone (a zone where the dislocation density is locally high) or by using other means, thereby enabling the structure of ferrite or the like, produced during cooling, to be refined.
Incidentally, in the conventional process, the grain diameter of the austenite before transformation is not more than 20 .mu.m, and also in the structure obtained by transformation, the grain diameter of the ferrite, for example, is not more than 20 .mu.m.
One of the hot-rolled steel sheets developed in the current process, which is a material required formability after punching (this material being used in, for example, strengthening components (members, wheels, etc.) of automobiles) is a high-strength hot-rolled steel sheet having an excellent stretch-flange ability (enlargeability). Such a steel sheet should have both a high strength as a strengthening member and workability. Up to now, high-strength steel sheets having a strength of up to 60 to 70 kgf/mm.sup.2 have been developed. As disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) Nos. 61-19733 and 1-162723, the steel sheets have a composite structure comprising a fine ferrite and a fine (in terms of packet size) low-temperature transformation phase (a fine pearlite, bainite or temper martensite). The term "packet" used herein is intended to mean a group of small units of a low-temperature transformation phase comprising a group of similar grain orientations which are identified by etching or the like. It is known that the local ductility, such as stretch-flange ability, is generally lowered when a phase having a hardness much greater than ferrite, such as cementite or martensite of large size, occupies, and attention has been paid particularly to homogenization and refinement (to not more than about 20 .mu.m) of the structure.
On the other hand, advances in casting techniques in recent years have enabling a thin cast strip having a thickness corresponding to that of the hot-rolled steel sheet to be produced by a twin roll casting process or the like. Since hot rolling used in the prior art can be completely omitted, this process has been studied as a cost-effective and energy-saving process mainly for producing a material for a cold-rolled steel sheet subjected to cold rolling/annealing. However, when the thin cast strip, as such, is regarded as a material corresponding to a hot-rolled steel sheet, since the austenite grain diameter is as large as about 1000 .mu.m, the structure mainly composed of ferrite also is generally likely to coarsen significantly. For this reason, the properties of the thin cast strip have hardly been studied.
The present inventors have aimed at the above thin cast strip and made studies with a view to producing a steel sheet having an excellent toughness or strength-ductility balance from the thin cast strip. As a result, they have succeeded in forming a fine bainite or Widmanstatten ferrite structure by cooling the material in an austenite region, i.e., in the temperature range of from 900.degree. to 400.degree. C., at a cooling rate of 1.degree. to 30.degree. C./sec to precipitate MnS, TiN, etc. which are utilized as nuclei in transgranular transformation, then conducting cooling in the temperature range of from 900.degree. to 600.degree. C. at a cooling rate of not less than 10.degree. C./sec to form the fine bainite or Widmanstatten ferrite structure composed mainly of the above precipitates. This was disclosed by the present inventors in Japanese Unexamined Patent Publication (Kokai) Nos. 2-236224 and 2-236228 and the like.
In the above-described thin cast strip, particularly Ti and B were added as a steel composition to form a precipitate of TiO, Ti.sub.2 O.sub.3, TiN or the like or a precipitate of BN, Fe.sub.23 (C-B).sub.6 or the like, which regulated ferrite produced in grain boundaries and, at the same time, contributed to nucleation of ferrite transformation, so that a fine ferrite or bainite structure could be formed.
Since, however, the above precipitates, which are utilized as transformation nuclei, are precipitated in an austenite region, they are likely to coarsen, so that the stretch-flange ability of the steel sheet with these hard precipitates dispersed therein is generally poor. For this reason, no detailed study has been made on techniques for improving the stretch-flange ability in the above-described thin steel sheet.
Accordingly, the present inventors have made new studies with a view to imparting stretch-flange ability to a steel sheet formed from the above-described thin cast strip.
The austenitic structure of hot-rolled steel sheets produced by the conventional process is so fine that it is generally difficult to impart stretch-flange ability to them. Specifically, the fine structure of the hot-rolled steel sheets unavoidably causes ferrite to be produced during cooling after hot rolling, which generally makes it difficult to provide a structure consisting of a low-temperature transformation phase alone, such as bainite, which is advantageous for the stretch-flange ability. For example, in the above-described Japanese Unexamined Patent Publication (Kokai) No. 61-19733, a low temperature transformation phase occupying not less than 50% of the structure is obtained with difficulty by adopting means such as use of somewhat high temperature in finish hot rolling to avoid refinement of austenitic structure and close control of cooling conditions. Further, Japanese Unexamined Patent Publication (Kokai) No. 1-162723 proposes the in situ formation of an intended structure which applies a high load on the process. Specifically, in this process, even after a martensite phase is formed by annealing in a two-phase region after hot rolling, tempering is carried out for the purpose of reducing a difference in hardness between the martensite and the ferrite.
The present inventors have made studies with a view to providing a thin steel sheet having an excellent stretch-flange ability and consisting of a low-temperature transformation phase alone through a smaller number of process steps than the conventional process and, as a result, have found that this object can be attained by cooling a steel sheet formed from the above thin cast strip at a particular cooling rate.
The above steel sheet is made on the premise that it is applied to strengthening members, and materials having a tensile strength of not less than 35 kgf/mm.sup.2 are contemplated.
Specifically, an object of the present invention is to provide a thin steel sheet having an excellent stretch-flange ability through a smaller number of process steps than the conventional process.
Another object of the present invention is to provide a thin steel sheet having both high strength and stretch-flange ability.
A further object of the present invention is to impart an excellent stretch-flange ability to a steel sheet formed from a thin cast strip.
CONSTITUTION OF INVENTION
The present inventors have made various studies on stretch-flange ability with a view to attaining the above-described objects and, as a result, have noticed that the austenitic structure of an as-cast thin steel strip, which has hitherto been ignored in the art, is very advantageous for the formation of a low-temperature transformation phase indispensable to a structure capable of imparting an excellent stretch-flange ability to the steel sheet.
Further, they have found that solidification of a molten steel followed by cooling, in a region where austenite is transformed to ferrite, at a predetermined cooling rate depending upon the compositions enables a desired very homogeneous low-temperature transformation phase, that is, a structure consisting of transgranular acicular ferrite, bainite, etc. alone, to be provided.
Specifically, the present inventors have succeeded in the formation of a structure consisting of a low-temperature transformation phase alone by adding no carbonitride forming element such as Ti and cooling as-cast solidified coarse austenite grains at a predetermined cooling rate to prevent the formation of intergranular ferrite and eliminate the precipitate, and a thin steel sheet having a very good stretch-flange ability while enjoying a high strength could be provided, for the first time, by virtue of the above structure.
The present invention has been completed based on the above finding, and the subject matter of the present invention is as follows.
The thin steel sheet according to the present invention is characterized by comprising, in terms of % by weight, 0.01 to 0.20% of C, 0.005 to 1.5% of Si, 0.05 to 1.5% of Mn and not more than 0.030% of S and optionally 0.0005 to 0.0100% of Ca and 0.005 to 0.050% of REM including Y with the balance consisting of Fe and unavoidable impurities, said thin steel sheet having a structure comprising at least one member selected from a transgranular acicular ferrite and a bainite having a packet size of 30 to 300 .mu.m in a proportion of not less than 95% of the structure and a sheet thickness in the range of from 0.5 to 5 mm.
The process for producing the above-described thin steel sheet is characterized by comprising the steps of: subjecting a steel comprising the above compositions to continuous casting into a thin cast strip having a casting thickness in the range of from 0.5 to 5 mm; cooling said thin cast strip from the temperature range of from the casting temperature to 900.degree. C. to the temperature range of from 650.degree. to 400.degree. C. at an average cooling rate of not less than V (.degree.C./sec) represented by the following formula (1) specified by C and Mn; and coiling the cooled strip at a temperature of not more than 650.degree. C.: EQU log V.gtoreq.0.5-0.8 log Ceq (.degree.C./sec) (1)
wherein Ceq=C+0.2 Mn.
In this case, the material may be lightly rolled in an in-line manner with a reduction ratio of not more than 20% for the purpose of breaking shrinkage cavities in the thin cast strip.